The present invention relates to apparatus and methods for controlling operation of electric motors, and more particularly to determining motor inertia in cases where a load is restricted to a limited range of travel.
Some industrial electric motors are operated by a motor drive which responds to a velocity command by applying electricity to the motor in a manner that causes the motor to operate at the commanded velocity. In a typical motor drive, the velocity command is compared to a measurement of the actual velocity of the motor to produce a commanded torque indicating how the motor's operation needs to change in order to achieve the commanded velocity. For example, to accelerate the motor a positive commanded torque is produced, whereas a negative commanded torque is required to decelerate the motor.
The amount of torque that is required to produce a given change in velocity is a function of the inertia of the motor and the mechanical apparatus being driven. The inertia in a typical industrial installation is determined and programmed into the motor drive upon commissioning the motor. Thus it is desirable to provide a mechanism for accurately estimating motor system inertia.
The traditional process for estimating the motor system inertia involves operating the motor through a linear acceleration/deceleration profile during a commissioning process. Here, for instance, a high target or test velocity may be specified, the motor and load may be accelerated to the test velocity and then decelerated to a zero or nominal velocity value. If the velocity changes at a constant rate, the motor torque is constant during both acceleration and deceleration and it is relatively straight forward to calculate the inertia. This is the case with drive systems that have regenerative capabilities (i.e. where the electric current induced in the stator coils during deceleration is able to flow unrestricted back into the DC supply bus for the motor drive).
In systems where there is no limit on motor rotations and load movement, the acceleration/deceleration inertia estimating techniques work well as the motor can be accelerated up to the high test velocity without regard to motor rotation limitations or load movement limitations.
Unfortunately, in some cases a motor and/or load may be restricted so that a motor cannot be driven to a high test velocity. For instance, in some systems a rotating motor may drive a linearly moving machine component (e.g., a transfer line) where the machine component moves between first and second limit positions at different ends of a range of load movement. Here, for example, a motor may only rotate 200 times while moving the linear machine component between the first and second limit positions. In the present example the motor may not be able to reach a high test velocity and then decelerate to a zero velocity within 200 rotations.
Where a motor cannot reach a test velocity specified by a commissioning procedure, the typical acceleration/deceleration inertia estimating technique typically is not performed thereby avoiding load and motor damage. Instead, in at least some cases, a person commissioning the system would have to manually measure motor operating characteristics during normal motor operation, calculate an inertia estimate based on the manual measurements and then enter the inertia estimate into the system for use by the system controller.